1. Field of the Invention
This invention relates in general to analytical schemes for transferring, preconcentrating, detecting and measuring target analytes from surfaces or interfaces using spectroscopic methods including optical spectroscopy.
2. Description of the Prior Art
Field measurement of suspected hazardous chemicals is a major challenge in applied analytical chemistry. Substances that would be of high interest include unknown residues on surfaces, for example a residue on a soldier's boot or on a military vehicle that is suspected of being a chemical warfare agent, or a spill of an unknown hazardous chemical presented to a first responder. One tool currently available to soldiers and first responders is a field-portable infrared spectrometer called a Hazmat ID (http://www.smithsdetection.com/HazMatID.php). The Hazmat ID is a ruggedized version of a commercially available Attenuated Total Reflectance (ATR) infrared spectrometer. The Hazmat ID allows a user to identify a number of solid and liquid samples in the field. However, the Hazmat ID requires that the sample is a nearly pure liquid or solid, and that a relatively large amount of pure sample is able to be placed and pressed up against the active sensing window. While this instrument works well for some applications, if a suspect residue is not isolatable or is a thin coating, the instrument will not be presented with adequate sample amounts to make a positive identification, potentially compromising mission operations.
Another example of a currently used field-portable Fourier Transfer Infrared (FTIR) spectrometer (http://wwvv.ahurascientific.com/chemical-explosives-id/products/trudefenderft/index.php) is an intelligently packaged ATR infrared spectrometer designed for measuring suspected target analytes by putting the optical sensing window in contact with the unknown chemical during analysis. However, because the sample is measured in situ, the suspected chemical must exist in high concentrations in order for a positive identification. Furthermore, positive identification is compromised if the substance is on a surface that contains bands in the same region of the infrared spectrum that obscure the measurement. In both of the above examples, adequate analysis of a trace residue on a surface would not be possible because the interfering optical signature of the surface itself would dominate the optical spectrum in which trace level measurement is desired. Another limiting factor for ATR-based measurements is that the target analyte must contact the ATR crystal surface, or minimally reside within 1-20 micrometers of the ATR crystal surface so as to be within the evanescent field extending beyond the crystal surface. Therefore, a roughened, porous or irregular (i.e., not flat) surface having features larger than these dimensions may contain a certain amount of analyte material that is not probed by a contact-based ATR measurement. Other spectroscopy tools and techniques are also employed to detect substances including techniques that do not examine optical characteristics of gathered substances. These other analysis tools experience similar limitations associated with the collection of samples for examination.
In the scientific literature, there are several examples of using cotton-based swabs to collect a target analyte and transfer the analyte to an ATR window for infrared spectroscopy. For example, Nel et. al, 2010 (Vibrational Spectroscopy, Volume 53, pp 64-70) describe using acetone soaked swabs to sample an adhesive residue from pottery. Following sampling, some of the acetone (which contains the adhesive) is deposited onto an ATR crystal and analyzed using infrared spectroscopy. However, in this case and others that describe transferring a bulk solvent, the swab is serving simply as a sorbent means to hold a bulk solvent, which is responsible for dissolving the target analyte. There is nothing novel about the collection device, and it is no different than using standard wet chemical laboratory techniques. These types of examples are significantly different from the proposed invention, which involves the use of designed materials with optimized physical (e.g. high surface areas) and/or chemical (molecular imprinting, tailored surface energy, surface charge, receptors with specific binding sites) and/or electromagnetic (charge, potential, flux) characteristics that result in high affinities for, and effective collection/concentration of, target analytes.
Murthy et al., 1985 (Applied Spectroscopy, Volume 39, Number 5, pp 856-860) and Murthy et. al, 1985 (Applied Spectroscopy, Volume 39, Number 6, pp 1047-1050) describe the transfer of polydimethylsiloxane (PDMS) and artificial body soil (ABS) from cotton fabric to a diamond ATR crystal, which is then analyzed using infrared spectroscopy. In these examples, cotton fabric was treated with ABS to test the efficacy of detergents on cotton fabric and PDMS to characterize a water repellant for the fabric. In both cases, pressure was applied to the cotton fabric to squeeze the ABS and PDMS onto the ATR crystal, much like squeezing water out of a sponge onto the optical window and removing the absorbent material. The pressure simply compresses voids found in the bulk fibrous structure of the cotton fabric, thus excluding the ABS and PDMS from the medium. This is significantly different from the present invention in that this invention uses specifically designed advanced materials with high affinities for analytes of interest including chemicals, biological compounds, and particles. The materials are designed to 1) collect, concentrate, and properly prepare a target analyte and 2) allow for subsequent analysis using spectroscopy without interference or to specifically transfer the chemical to another material that allows for analysis using spectroscopy without interference.
Finally, U.S. Pat. No. 7,808,632 discusses using absorptive materials for gas phase analysis. The present invention is significantly different because the materials used are designed for analysis of solids and liquid residues. What is needed is a better tool and process for capturing analytes in satisfactory concentration and for transferring them for measurement.